Nonaqueous electrolyte secondary batteries including lithium ion secondary batteries provide a high voltage and a high capacity, and therefore such batteries are widely used as power sources for various portable devices. In recent years, their applications are spreading to medium and large size applications such as power tools including electric tools, electric vehicles and electric bicycles.
Under the circumstances, there has been a demand for a higher capacity of such a lithium ion secondary battery, and for the purpose, research and development of electrode active materials exhibiting a high charge-discharge capacity has proceeded. Among them, for the negative electrode active material, materials such as silicon (Si) and tin (Sn) that are capable of occluding and releasing more lithium (ions) have received attention in place of carbonaceous materials such as graphite that has been employed in conventional lithium ion secondary batteries. In particular, SiOx having a structure where ultrafine particles of Si are dispersed in SiO2 has been known also as having excellent load characteristics and the like.
However, since SiOx experiences a great change in volume as a result of charge and discharge of a battery, in a battery using this, the battery characteristics may deteriorate rapidly due to repetition of charge and discharge.
Research and development for solving this problem have been done. For example, Patent document 1 proposes a nonaqueous secondary battery having high capacity and ensuring high battery characteristics by combining a negative electrode using graphite in conjunction with SiOx and a positive electrode using a Li-containing transition metal oxide of a specific formulation and by further controlling potential of the negative electrode at the time of discharging to 2.5 V.
In an alternative technique, it is also known to adjust the construction of the nonaqueous electrolyte used for a lithium ion secondary battery, thereby improving the battery characteristics of the lithium ion secondary battery (for example, Patent documents 2-4).